The present invention relates to a method of applying a varnish, including a solvent or free of any solvent, on a component which may in particular be a vehicle headlight reflector.
The majority of basic compositions of varnishes which are applied on components of the type that include vehicle headlight reflectors made from solid moulding mixtures such as BMC, CIC and so on, contain large quantities of solvents. The presence of these solvents is judged necessary in order to reduce the viscosity of the products applied and to improve the application performance, for example by spraying. During the spraying step and then during the period of removal of the solvents, the solvents evaporate wholly or partly. The time during which trapped air bubbles are able to escape and the varnish film is able to spread, depends on the viscosity of the products used and the nature and residual concentration of the solvents. The longer is the solvent removal time, the greater are the risks of increased flowing out of the varnish on the surface of the components, so giving rise to numerous rejects.
The Applicant has accordingly set the particular objective of obtaining excellent performance of the application of a varnish on components of the vehicle headlight reflector type, while limiting its tendencies to xe2x80x9cflow outxe2x80x9d on the surface after application and without reducing the qualities of appearance in the base coating film.
This object is achieved by the present invention, which provides a method of applying a varnish, whether or not it includes a solvent, on a component, in particular a vehicle headlight reflector, characterised in that it comprises the following steps, namely: (i) selecting a varnish having a viscosity at ambient temperature in the range from about 500 mPa.s to 2000 mpa.s, and a viscosity at the application temperature which is lower than about 200 mPa.s, with a viscosity which is substantially independent of variation in the temperature within the range of application; (ii) heating the surface of the component before the step of moistening the varnish at a selected temperature xcex8p1; (iii) heating the varnish at the moment of application to a selected temperature xcex8v; and (iv) during the spreading step, maintaining the component at a selected temperature xcex8p2 for a selected time tp2.
The varnishes, whether or not they contain a solvent, generally consist of resins the viscosity of which is directly linked to temperature.
The method according to the invention, which may be performed equally well, whether or not the varnish contains a solvent, through the choice of appropriate varnishing temperatures, enables excellent application performance to be guaranteed, in particular by spray application. The selection of temperatures for the component to be varnished, before the application step and during spreading, also enables, in particular, the wettability of the varnish and its mobility to be controlled and optimised. In practice, the viscosity of the varnish achieves its optimum value at these selected temperatures. When applied on a hot surface, excellent wettability is obtained on the one hand, and removal of bubbles from the varnish is accelerated on the other hand. On cooling, viscosity increases rapidly so that the risks of flowing out are reduced.
Preferably, in the method according to the invention, step (ii) consists in heating by radiation, convection or conduction, for example using a forming tool, and step (iv) consists in heating by radiation or convection. This type of heating enables correct spreading to be obtained for the smoothest possible optical surface.
In a first embodiment in which the varnish includes a solvent, xcex8v is about 20 to 40xc2x0 C., xcex8p1 is about 20 to 50xc2x0 C., and xcex8p2 and tp2 about 20 to 50xc2x0 C. and about 0.5 to 3 min, respectively.
By comparison with methods with traditional solvents, in which the solvent conventionally represents about 20% to 70% by weight of the overall varnish composition, in the method of the invention the solvent does not represent more than 10% by weight of the overall composition of the varnish used. Solvent removal times in the method according to the invention are therefore diminished overall, which greatly reduces the cost.
In a second embodiment of the method of the invention, the varnish being solvent free, the temperature xcex8v is about 40 to 60xc2x0 C., the temperature xcex8p1 is about 70 to 110xc2x0 C., the temperature xcex8p2 is about 50 to 90xc2x0 C., and the time tp2 is about 0.5 to 1.5 min.
One of the major advantages of this embodiment is first that the varnish formulations are simplified and it is performed without solvents. Apart from their volatile nature, solvents have a toxicity level which may be more or less acceptable, and are a significant fire risk. These constraints usually make it necessary to have in place installations for treatment of the volatiles, together with fire fighting equipment in all the zones concerned. This embodiment therefore enables economies of investment and exploitation to be achieved by simplification of the plant, since the application zone is equipped with fire prevention means because of the need for cleaning the material. It also has the advantage of putting the plant into conformity with even stricter regulations in regard to the environment.
The complementary or alternative features of the method according to the invention are as follows:
The varnish is applied by spraying under pressure (xe2x80x9cairlessxe2x80x9d), with a compressed air or electrostatic spraygun; preferably, the spraygun is equipped with a thermo-regulated flow loop, which enables the viscosity of the varnish to be reduced and enables the temperature xcex8v and the varnish flow to be constant at the moment of spraying by the spraygun. Without the said loop, temperature control problems would arise because the spraygun works intermittently; the thickness of the varnish is obtained in a single pass of the spraygun: the varnish is deposited perfectly on the workpiece and no retouching is necessary; when the varnish applied by the spraygun is a solvent-free varnish, this enables major variations to be obtained in the thickness which are compatible with the complex geometry of the surface being covered, and this increases the robustness of the method; with identical thickness, two passes of the spraygun are in general necessary in the case where the varnish does have a solvent.
The varnish is a UV varnish: compared to so-called thermal varnishes, UV varnishes save a certain amount of time, because once the required appearance has been obtained the latter is xe2x80x9csetxe2x80x9d practically instantaneously, compared with times of the order of 5 to 10 minutes which are necessary with thermal varnishes. During these setting times, the atmosphere of the plant must remain very clean and xe2x80x9cdust freexe2x80x9d.
When applied on a component such as a BMC component, the method includes a step of pre-treatment with UV radiation at about 1 to 4 J/cm2 with a maximum power of about 130 to 250 mW, measured in the UVA band and in a plane substantially at right angles to the mean radiation of the UV illumination zone. Effectiveness of treatment of the surface of the workpiece, and adherence of the varnish on the BMC are thus guaranteed.
The method includes a step of polymerisation step with UV radiation at between about 4 and 8 J/cm2, and preferably about 2 to 4 J/cm2 with a maximum power of about 80 to 200 mW measured in the UVA band and in a plane substantially at right angles to the mean radiation of the UV illumination zone. In this way a brilliant spread-out surface, capable of being metallised, is obtained.
The orientation of the light radiation from each emitter, or the putting of the workpiece in motion, are optimised, which enables homogeneous illumination to be guaranteed regardless of the complexity of the exposed surface; in practice, the emitters are inclined so as to take the orientation of the faces of the component into account;
In the UV polymerisation step, in which varnish temperature xcex8p2 is high, the varnish is more reactive. This increase in reactivity enables the number of UV emitters to be substantially reduced, while zones which are generally difficult to expose are polymerised;
The method includes a step of rotating the component about a horizontal axis in the course of the spreading and/or polymerising step, which enables flowing-out effects to be avoided and also enables the maximum thickness of the deposited layer to be increased on application, all this regardless of the form of the component being treated. In this way the soundness of the application method is increased.
The present invention will now be described in more detail, as to its general features, and then with the aid of an example of an embodiment given by way of non-limiting illustration.
In general terms, after injection of a component of thermosetting material, a method of applying a UV varnish comprises the conventional steps of washing the workpiece in water, drying, UV pre-treatment, application of the heated varnish, or a so-called xe2x80x9cmoisteningxe2x80x9d step, xe2x80x9cspreadingxe2x80x9d, UV polymerisation, and metallisation.
The method of the invention may be applied to any workpiece capable of supporting transient temperatures up to about 110xc2x0 C.
In the case of reflectors for vehicle headlights, there may be mentioned, by way of components in thermosetting material that can be used in the method of the invention, mixtures for moulding which are solid, or BMC, meaning xe2x80x9cbulk moulding compoundsxe2x80x9d (or their variants such as CIC, which means xe2x80x9ccontinuous impregnated compoundsxe2x80x9d, TMC which means xe2x80x9cthick moulding compoundsxe2x80x9d, or again, SMC which means xe2x80x9csheet moulding compoundsxe2x80x9d), which are unsaturated polyester resin based compositions reinforced with glass fibre. Components of phenolic, vinyl or epoxy resins, or again urea-formol melamines, can also be suitable.
As a UV varnish that can be used in the method of the invention, mention may be made of the compositions which conventionally include an acrylic resin or a mixture of acrylic resins, with the optional addition of one or more solvents.
The acrylic resins used in the method of the invention may comprise one or more of the following compositions:
Polymers or co-polymers of esters of acrylic acid such as multi-functional acrylate oligomers, for example of the epoxy acrylate, aliphatic or aromatic urethane acrylate, polyester acrylate, or acrylic acrylate types; these compounds are typically viscous liquids having at 25xc2x0 C. a viscosity in the range from around a few thousands to around more than a million centipoise (cP), having in general 2 to 6 acrylate groups per molecule and possessing a molecular weight in the range from about 500 to about 20,000.
Multi-functional acrylate monomers having in general from 1 to 4 acrylate groups per molecule and a molecular weight in the range from about 150 to about 500, their viscosity at 25xc2x0 C. being in the range from about 5 to about 200 cP.
Hexanedioidiacrylate (HDDA), tripropyleneglycoldiacrylate (TPDGA), triethyleneglycoldiacrylate (TEGDA), and dipropyleneglycoldiacrylate (DPGDA), and monomers of the acrylic acid type.
One or more photo-initiators of the arylketone type, for example benzophenone, hydroxycyclohexylphenylketone (HCPK), and 2,2-dimethoxy-1,2-diphenylmethane-1-one.
One or more tensioactive agents of the alkylpolysiloxane type, such as dimethylpolysiloxane or methacrylatesiloxane, a polyethersiloxane or polyestersiloxane.
Other additives may be added to the foregoing compounds, such as for example one or more anti-foaming agents, adhesion promoters, thixotropic agents, stabilisers, colouring agents, etc.
When they are present in the performance of the method of the invention, the appropriate solvents are for example solvents of an ester type, such as for example ethyl acetate, vinyl or butyl acetate, or again, of the glycol ester type such as methoxypropyleneacetate (MPA), or of the ketone type, such as for example methylisobutylketone (MIBK) or methylethylketone (MEK), or, again, of the alcohol type such as butylic alcohol or, finally, of an aromatic type such as toluene or xylene for example.